1. Field of the Invention
This invention relates to liquid ink compositions comprising a charge director, a grafted co-polymer having at least one acidic group or at least one basic group, and a dispersant having at least one basic group or at least one acid group, respectively. In particular, this invention relates to a liquid ink which exhibits improved dispersion stability and improved chargeability when used in any imaging process, including but not limited to ink transfer processes, ionographic processes, electrographic processes and electrophotographic color printing processes or proofing processes.
2. Background of the Art
Liquid inks are widely used in a variety of imaging and printing processes, for example offset, bubble jet, ink jet, intaglio, rotogravure, electrographic, and electrophotographic printing. Many characteristics desired in pigment dispersions for liquid inks are the same for each of the respective processes even though the final ink formulations may be substantially different. For example, the stability of the pigment dispersion on the shelf, under shear conditions, and under high voltage fields is an important consideration regardless of the final use of the liquid ink. The art continuously searches for more stable pigment dispersions to provide more flexibility in ink formulations to provide better efficiency and waste reduction in the various printing processes.
In electrophotographic applications, which include devices such as photocopiers, laser printers, facsimile machines and the like, liquid inks are referred to as liquid toners or developers. Generally, the electrophotographic process includes the steps of forming a latent electrostatic image on a charged photoconductor by exposing the photoconductor to radiation in an imagewise pattern, developing a temporary image on the photoconductor by contacting the photoconductor with a liquid ink, and finally transferring the temporary image to a receptor. The final transfer step may be performed either directly from the photoconductor or indirectly through an intermediate transport member. The developed image is usually subjected to heat and/or pressure to permanently fuse the image to the receptor.
Liquid inks typically comprise an electrically insulating liquid that serves as a carrier for a dispersion of charged particles, known as toner particles. These toner particles are composed of at least a colorant (e.g. pigment or dye) and a polymeric binder. A charge control agent is often included as a component of the liquid developer to regulate the polarity and magnitude of the charge on the toner particles. Liquid inks can be categorized into two primary classes. For convenience, the two classes will be referred to as conventional liquid inks and organosol inks.
One problem in formulating liquid inks is the difficulty in obtaining liquid inks that have excellent dispersion stability. It is always desirable to obtain liquid ink compositions having excellent dispersion stability.
An important consideration in formulating liquid inks is the chargeability of the liquid ink. It is desirable for a liquid ink to have a high chargeability so that it can acquire enough quantity of charge to be forced under an electrical field to migrate and then to plate upon the imaged areas on the photoreceptor. The chargeability of a liquid ink is measured by its conductivity and its mobility. In general, liquid inks with high ink conductivity and high ink mobility are desirable.
This invention provides a liquid ink with a novel composition that exhibits increased dispersion stability and improved chargeability.